This invention relates to a method and plant for separating air. In particular, it relates to the separation of a krypton-xenon concentrate from air.
The main components of air are oxygen and nitrogen. Oxygen and nitrogen are separated from air by rectification at cryogenic temperatures. The air is pre-purified by adsorption to remove impurities such as water vapour and carbon dioxide which freeze at cryogenic temperatures. The purified air is cooled to a temperature suitable for its rectification. The rectification is typically conducted in a double rectification column comprising a higher pressure column, a lower pressure column and a condenser-reboiler placing the higher pressure column in heat exchange relationship with the lower pressure column. The higher pressure column serves to provide liquid nitrogen reflux for the lower pressure column and to produce an oxygen-enriched air fraction which is separated in the lower pressure column. If an argon product is required, a stream of argon-enriched oxygen vapour, typically containing some 5 to 15% by volume of oxygen, is withdrawn from an intermediate region of the lower pressure column and is separated in a further rectification column.
Whereas nitrogen, oxygen and argon make up more than 99% by volume of ambient air, the proportions of krypton and xenon in it are small indeed. Thus, each million volumes of air contains about 1.1 volumes of krypton but only 0.08 volumes of xenon. Nonetheless, krypton and xenon are both valuable industrial products. Both gases are used in lighting in view of their fluorescent properties. In addition, xenon is used in flash photography and X-ray photography. Xenon also has anaesthetic properties and was used in the former Soviet Union as an anaesthetic gas for maintaining a state of anaesthesia.
Because krypton and xenon have a low volatility in comparison to oxygen, they tend to accumulate in the liquid oxygen fraction which is obtained at the bottom of the lower pressure column of the double rectification column during separation of air to form an oxygen product. Nonetheless, the proportion of krypton and xenon in this bottom fraction is still very small and a number of purification steps need to be formed in order to obtain relatively pure krypton and xenon products. The first of these purification steps involves continuously withdrawing a stream of krypton-xenon concentrate from the bottom of the lower pressure column and separating it in a further rectification column which is provided with both a reboiler and a condenser. According to W H Isalski at page 97 of his textbook "Separation of Gases" (Clarendon Press, Oxford (1989)) the bottom fraction obtained from the further concentration of the liquid oxygen in krypton and xenon still only contains from 0.1 to 0.2% by volume of krypton and xenon. The bottom fraction is subjected to so-called secondary and tertiary separations in order to provide krypton and xenon products. Frequently, a significant proportion of the krypton/xenon content of the air is lost in gaseous oxygen product. (Even more would be lost were oxygen product to be taken in liquid state).
There is therefore a need to improve upon the conventional method and plant for obtaining the krypton-xenon product by making it possible both to achieve relatively easily a greater proportion of krypton and xenon in the concentrate and to improve the recovery of krypton and xenon. It is the aim of the method and plant according to the invention to meet this need.